Compositions and methods for high temperature stable catalysts

ABSTRACT

Catalysts having good high temperature stability which are particularly useful for selected high temperature reactions are disclosed as are methods for their preparation and use. The catalytically-active materials include platinum group metal deposited on a catalytic slip or composite which contains a mixture of alumina and a mixture of IVB and selected VIB metal oxides. The slips or carrier compositions are calcined at a temperature of at least 850° C. before deposition of platinum group metal and characterized by having a surface area of at least 20 m 2  /g after calcination at a temperature of 1200° C. for two hours.

This is a division of application Ser. No. 423,095, filed Dec. 10, 1973,now U.S. Pat. No. 3,945,946, issued on Mar. 23, 1976.

The present invention relates to catalyst compositions and methods fortheir preparation and use. In particular, this invention relates tocatalyst compositions characterized by high stability therebymaintaining good catalytic activity.

Catalyst compositions exhibit a relatively high surface area per unitweight to allow the largest amount of reactants to contact the catalyst.Additionally, high surface area is important when the catalystcomposition contains a precious metal such as platinum because of thecost of the metal and because of the dispersion required to preventundue metal crystallite growth. It is desirable to retain this highsurface area for long periods of use under severe conditions which mightinclude reaction temperatures of 1200° C. or higher.

Alumina is an excellent and relatively economical carrier or support formany catalysts. Many crystalline forms of alumina, for example, chi,kappa, gamma, delta, eta, and theta, exhibit a very high surface area inrelation to their weight. A serious drawback of alumina as a catalystcarrier, however, is its transition temperature of about 1000°-1200° C.to the alpha form which results in a substantial reduction of thesurface area. It is thus extremely desirable to stabilizealumina-containing catalyst compositions based on high surface areaaluminas to substantially prevent the transition to the low surfacealpha form with a consequent loss in activity.

It is therefore an object of this invention to provide catalystcompositions, as well as methods for their preparation and use, whichexhibit high temperature stability. Other objects and advantages willappear as the description proceeds.

Broadly, the catalyst composition of this invention includes acatalytically-active, calcined composite characterized by a surface areaof at least 20 square meters per gram (m² /g) after calcination for twohours at a temperature of 1200° C., said composite comprising or being acomposite of alumina and a mixture of two metal oxide components whereinthe first metal oxide component is selected from the group consisting ofchromium, tungsten, and mixtures thereof and the second metal oxidecomponent is selected from the group consisting of Group IVB metals andmixtures thereof. In preparing the catalyst composition, the compositeis first calcined at a temperature of at least 850° C., and then acatalytically-effective amount of a platinum group metal is added to thecomposite. A catalyst composition prepared in accordance with thisinvention exhibits high temperature stability and therefore catalyticactivity in a number of high temperature reactions, particularly hightemperature combustion reactions.

The composite is formed by the calcination of an intimate admixture ofan aluminum compound and two metal oxide components wherein the firstcomponent is selected from the group consisting of chromium, tungsten,and mixtures thereof and the second component is selected from the groupconsisting of Group IVB metals and mixtures thereof. Preferably, forcertain methods of preparation, the aluminum compound is alumina. Thesecompounds, as indicated, if not already in oxide form must be capable offorming or yielding their respective oxides upon calcination in air(oxygen) at a temperature of at least 850° C. The metal oxide or oxidesmixture may be considered as a high temperature stabilizing componentfor the alumina.

The relative amounts of alumina to the metal oxide stabilizing componentare governed largely by empirical criteria. While it is not desired thatthis invention be limited by the following theory, a brief statement mayprovide a helpful framework to further elucidate the invention. It isthought that the addition of the stabilizing component to the alumina oralumina precursor and calcination of the mixture at a temperature of atleast 850° C. converts any of the non-oxide compounds to oxides andallows the stabilizing component oxides to enter the alumina lattice andprevent or substantially reduce subsequent transition to alpha alumina.

All surface areas throughout the specification and the appended claimsare measured by the B.E.T. or equivalent method. The terminology used todescribe the metals herein, that is, the Group IVB metals, is theterminology used in association with the common long form of thePeriodic Table of Elements. Thus the Group IVB metals are titanium,zirconium, hefnium, and thorium.

The catalyst composition may also contain a minor amount of otheringredients, up to about 5 percent by weight of the composite, which mayserve as promoters, activators, or other purposes, for oxidation orreduction reactions. Such ingredients may include, for examplemanganese, vanadium, copper, iron, cobalt, and nickel usually as themetal oxide or sulfide.

The calcined composite may be formed to any desired shape such as apowder, beads, or pellets. This shaping or fabricating is accomplishedbefore calcination to promote particle adhesion. After calcination, aplatinum group metal is added to the composite. Additionally, thecomposite can be applied or deposited on a relatively inert support orsubstrate and the platinum group metal then added, or the catalystcomposition can be applied or deposited onto the inert support.

For compositions made in accordance with this invention, the compositegenerally comprises about 80 to 95 weight percent alumina. The Group IVBmetal oxide, may be present in about 2 to 15 weight percent of thecomposite, preferably about 5 to 15 weight percent. The chromium and/ortungsten oxide may be present in about 2 to 15 weight percent,preferably about 5 to 15 weight percent of the composite. The mixture ofGroup IVB metal oxides and chromium and/or tungsten oxides may bepresent in about 5 to 20 weight percent, preferably about 5 to 15 weightpercent of the composite. If the amount of alumina is too low, theresulting composite will not provide enough surface area to providecatalytic activity. If more alumina is present than stated, it may notbe stabilized sufficiently and will lose surface area in the transitionto the alpha form.

Generally, to provide the advantages of this invention, it is necessaryfor the stabilizing component to be in intimate association with thealumina during pre-calcining. An intimate admixture may be achieved, forexample, by forming a slurry of alumina with water soluble compounds ofthe stabilizing components. Where desired, hydrated alumina, such asaluminum trihydrate is admixed with aqueous solutions of the mixture ofthe metal salts of this invention to permit sorption of the stabilizingcomponents by the alumina. The solids are then recovered from the slurryand calcined to provide the mixed oxide composite. The particulatealumina is preferably in finely divided or colloidal form to providemaximum sorption area. For example, finely divided freshly precipitatedaluminum trihydrate having a particle size of 70 percent to 90 percentsmaller than 325 mesh is useful. When large particle size alumina isused, the sorption of the stabilizing components from solution andsubsequent calcination will provide at least a stabilized outer portionof the alumina.

Another method of preparing intimate admixture of alumina andstabilizing components is to coprecipitate all of the components,including the alumina, from aqueous solutions. Various methods ofcoprecipitation are suitable. Such methods include, for example, surfaceadsorption where one or more components in ionic form are sorbed on thesurface of a precipitating solid; and inclusion, in which thecoprecipitated compound or compounds have dimensions and a chemicalcomposition which will fit into the crystal structure of a precipitatingsolid without causing appreciable distortion.

In coprecipitation, a suitable precipitant, usually a base, is added toan aqueous solution of the compounds. This can also be down byconcurrent addition of both the precipitant and the compound solution toa vessel containing water. Preferably the precipitant is selected suchthat undesirable or unnecessary compounds are volatilizable anddecomposable upon calcination at 850° C. or above, or removable bywashing or extraction. The precipitant is capable of initiating andcompleting essentially simultaneous coprecipitation of the components.Suitable precipitants are ammonium compounds such as ammonium hydroxideor ammonium carbonate as well as other hydroxides and carbonates of thealkali metals.

The precipitant may be in dilute or concentrated aqueous solution. Therapidity of addition of the precipitant and the degree of agitation usedwill vary depending upon the precipitate desired. Dilute precipitantsolutions, slow addition, and vigorous agitation generally favor acoarser precipitate. The temperature during the addition of precipitantmay be from about 0° to 90° C. Higher temperatures generally produce acoarser precipitate. The precipitant is added until a pH of about 5 to9.0 is reached. At this time the coprecipitated mixture is recoveredfrom the slurry, washed if desired, and digested or recrystallized ifdesired.

The intimate admixture of alumina and stabilizing components arecalcined at a temperature of at least about 850° C., preferably about900° to 1200° C., but not at such a high temperature or for such a longperiod of time to unduly sinter the composite. The conditions of thecalcination are such as to provide a catalytically-active compositehaving a relatively high surface area of at least about 25 square metersper gram, and preferably at least about 75. Calcination is preferablyconducted while the admixture is unsupported and in free-flowingcondition. This is preferable for economic reasons and to prevent unduesintering.

Calcination in air to form the composite, and prior to the addition of aplatinum group metal, is an integral part of the subject invention. Itis found that an intimate admixture of the stabilizing components andthe alumina is stable when calcined at such temperatures before anyfurther preparative steps are performed. Since both the alumina and thestabilizing components are intimately admixed, the concurrent heating inclose association substantially reduces any undesirable aluminatransitions. Additionally, calcination before deposit on an inertsubstrate promotes adhesion of the calcined composite to the substratethus allowing the use of higher space velocities with the finishedcatalyst composition with less chance of erosion. Further, calcinationsubstantially reduces the possibility of reaction of the stabilizingcomponent and alumina component with the substrate. Any such reactionsbetween the alumina and the substrate promotes the formation of inactiveforms of alumina thereby reducing its surface area and activity. If thestabilizing component were to react with the substrate, it would reducethe effective amount of this component available for stabilization. Afurther advantage of such calcination is economic because less heat insmaller furnaces is required to calcine the resulting powder compositebefore it is placed on an inert support. Further, it is essential thatthe calcination is conducted before the addition of a platinum groupmetal component to prevent loss of such component by occlusion.

Suitable aluminum-containing compounds are alumina, the gamma, eta,kappa, delta, and theta forms of alumina and for coprecipitation, thewater soluble aluminum compounds such as salts, for example, thealuminum halides, aluminum nitrate, aluminum acetate, and aluminumsulfate.

The Group IVB metal oxides, i.e., the oxides of titanium, zirconium,thorium and hafnium, are added to the alumina in the form of their watersoluble precursors. Thus, for example, water soluble IVB metal saltssuch as the nitrates, acetates, halides, and sulfates and the like mightbe employed. Suitable water soluble compounds are Zr(NO₃)₄, ZrCl₄,Zr(SO₄)₂, ZrOCl₂, Ti₂ (C₂ O₄)₃, and HfOCl₂.

Water soluble compounds of chromium and tungsten which can be used are,for example, chromium acetate, chromium nitrate, chromium halides,chromium oxide (chromic acid), chromium oxalate, and complexes ofchromium such as chloropentamine chromium chloride, tungsten halides,tungsten oxy-salts, such as tungsten dioxydichloride, ammoniumtungstate, and the like.

A platinum group metal is added to the calcined composite to form thecatalyst compositions of this invention, which are found to be effectivefor long time high temperature reactions. Such metals are usually addedor incorporated in amounts sufficient to provide significant activity.The platinum group metals useful are platinum, ruthenium, palladium,iridium, and rhodium. The choice of metal, metal combinations or alloysis governed largely by activity, specificity, volatility, deactivationby specific components included with the reactants, and economics.

The quantity of platinum group metal added to the calcined compositedepends first on design requirements such as activity and life andsecond on economics. Theoretically, the maximum amount of such metal isenough to cover the maximum amount of surface available without causingundue metal crystallite growth and loss of activity during use. Twomajor competing phenomena are involved in such surface treatment. It isdesirable to completely cover the substrate surface to provide thegreatest amount of platinum group metal coverage, thereby obtainingmaximum activity, but if the surface were to be completely covered, suchcoverage would promote growth between adjacent crystallites, whichgrowth would then decrease the surface area and greatly reduce activity.A balance of maximum coverage coupled with proper dispersion thus mustbe achieved to formulate a practical catalyst. An ancillaryconsideration in relation to the amount of platinum group metal is theallowable size of the catalyst housing. If the size is small, the amountof platinum group metal component used is preferably increased withinthe above-described limits. For example, for automobile exhausttreatment, the allowable size is relatively small, especially if unitaryhoneycomb type supports are used and a higher loading may be desirable.Economics, of course, dictates the use of the least amount of platinumgroup metal component possible while accomplishing the main objective ofpromoting the reaction. Generally, the amount of platinum group metalused is a minor portion of the catalyst composite and typically does notexceed about 20 weight percent of the calcined composite. The amount maybe about 0.1 to 20 percent and is preferably about 0.2 to 10 percent toeconomically maintain good activity with prolonged use. Thesepercentages are based on the weight of the calcined composite. If thecomposite is used on an inert substrate, the composite may be, forexample, about 10 percent of the weight of the substrate and the percentweight of platinum group metal in relation to the total weight ofsubstrate and composite will be correspondingly less.

During preparation of the catalyst composition, various compounds and/orcomplexes as well as elemental dispersions of any of the platinum groupmetals may be used to achieve deposition of the metal on the composite.Water soluble platinum group metal compounds or complexes may be used.The platinum group metal may be precipitated from solution, for example,as a sulfide by contact with hydrogen sulfide. The only limitation ofthe carrier liquids is that the liquids should not react with theplatinum group metal compound and be removable by volatilization ordecomposition upon subsequent heating and/or vacuum, which may beaccomplished as part of the preparation or in the use of the completedcatalyst composition. Suitable platinum group metal compounds are, forexample, chloroplatinic acid, potassium platinum chloride, ammoniumplatinum thiocyanate, platinum tetrammine hydroxide, platinum groupmetal chlorides, oxides, sulfides, and nitrates, platinum tetramminechloride palladium tetrammine chloride, sodium palladium chloride,hexammine rhodium chloride, and hexammine iridium chloride. If a mixtureof platinum and palladium is desired, the platinum and palladium may bein water soluble form, for example, as amine hydroxides or they may bepresent as chloroplatinic acid and palladium nitrate when used inpreparing the catalyst of the present invention. The platinum groupmetal may be present in the catalyst composition in elemental orcombined forms, e.g., as an oxide or sulfide. During subsequenttreatment such as by calcining or upon use, essentially all of theplatinum group metal is converted to the elemental form.

While these catalyst compositions are useful in many reactions, they arenot necessarily equivalent in all processes nor are those which areuseful in the same process necessarily exactly equivalent to each other.

While it is not essential, the catalyst compositions of this inventionpreferably have a relatively catalyticallyinert support or substrate.The supports which can be employed in this invention are preferablyunitary, skeletal structures of relatively large size, e.g., honeycombs.However, smaller particle forms may be used, e.g., pellets or spheres.The size of these pellets can be altered depending upon the system, itsdesign and operating parameters in which they are to be used, but mayrange from about 1/64 to 1/2 inch, preferably 1/32 to 1/4 inch, indiameter; and their lengths are about 1/64 to 1 inch, preferably about1/32 to 1/4 inch.

When a support is used, the calcined composite is generally present in aminor amount of the total catalyst composition, which is usually about 2to 30 weight percent preferably about 5 to 20 weight percent, based onthe total weight of the composite and support. The amount used dependson economics, size limitations, and design characteristics.

These supports whether of the unitary-skeletal type or pellets arepreferably constructed of a substantially inert, rigid material capableof maintaining its shape and strength at high temperatures, for example,up to about 1800° C. The support typically has a low thermal coefficientof expansion, good thermal shock resistance, and low thermalconductivity. While a support having a porous surface is preferred, thesurface may be relatively non-porous, but in such event it is desirableto roughen the surface to improve adhesion of deposited compositions.

The support may be metallic or ceramic in nature or a combinationthereof. The preferred supoorts, whether in skeletal or other form, arecomposed primarily of refractory metal oxide including combined oxideforms, e.g., aluminosilicates. Suitable support materials includecordierite, cordierite-alpha alumina, silicon nitride, silicon carbide,zircon-mullite, spodumene, alumina-silica-magnesia, and zirconiumsilicate. Examples of other suitable refractory ceramic materials aresillimanite, magnesium silicates, zircon, petalite, alpha-alumina, andaluminosilicates. Although the support may be a glass ceramic, it ispreferably unglazed and may be essentially entirely crystalline in formand marked by the absence of any significant amount of glassy oramorphous matrices. Further, the structure may have considerablyaccessible porosity, preferably having a water pore volume of at leastabout 10 percent. Such supports are described in U.S. Pat. No.3,565,830, herein incorporated by references.

The geometric, superficial, or apparent surface area of the skeletal orhoneycomb type supports, including the walls of the gas flow channels isgenerally about 0.5 to 6, and 2500, cross-sectional preferably 1 to 5,square meters per liter of support. This surface area is sufficient fordeposition of a satisfactory quantity of the composite or the finishedcatalyst composition. The plurality of channels, about 100 to 1500,preferably 150 to 500 per square inch of crosssectional area, may bedistributed across the entire face of the structure and frequently theydefine an open area in excessof 60 percent of the total area of thesupport. The walls must be thick enough to provide rigidity andintegrity to the structure while maintaining good apparent surface area.The wall thickness is thus in the range of about 2 to 25 mils. The flowchannels can be of any shape and size consistent with the desiredsuperficial surface area and should be large enough to permit relativelyfree passage of the gaseous reaction mixture; preferably the length ofthe channels is at least about 0.1 inch to insure sufficient contact orresidence time to cause the desired reaction. Although the channels aregenerally parallel, they may be multi-directional and may communicatewith one or more adjacent channels.

In one manner of preparing structures provided with catalystcompositions of this invention, an aqueous slurry of the essentiallywater insoluble calcined composite of alumina and stabilizing componentis contacted with the support. The solid content of the slurry forms anadherent deposit on the support, and the resulting supported compositeis dried or calcined for a second time at a temperature which provides arelatively catalytically-active product. The second drying orcalcination takes place at a temperature low enough to prevent unduesintering of the mixture. Suitable calcination temperatures aregenerally about 300°-700° C. to insure catalytic activity without unduesintering, preferably about 400°-600° C. After this second calcinationthe coating on the support has a surface area of at least about 75s.m.p.g. Lower temperatures can be employed to dry the composite if thesecond calcination is not performed.

After the coated support is dried or calcined, a platinum group metalcomponent is added to enhance the catalytic activity of the composite.The platinum group metal may be added to the coated support in themanner previously described. Preferably, this addition is made from anaqueous or other solution to impregnate or deposit the platinum groupmetal component on the coated support.

After addition of the platinum group metal, the resulting structure isdried and may be calcined for a third time under conditions whichprovide a composition having characteristics that enhance selectedreactions. This final calcination stabilizes the completed catalystcomposition so that during the initial stages of use, the activity ofthe catalyst is not materially altered. The temperature of this finalcalcination must be low enough to prevent substantial sintering of theunderlying coating which would cause substantial occlusion of theplatinum group metal component. Thus the calcination may be conducted attemperatures of about 300°-700° C., preferably about 400°-600° C. Analternative method of making the catalyst compositions of this inventionif a relatively inert support is used involves adding the platinum groupmetal component to the calcined composite before the composite isdeposited on the support. For example, an aqueous slurry of the calcinedcomposite can be prepared and the platinum group metal component addedto the slurry and mixed intimately therewith The platinum group metalcomponent can be in the form already described and may be precipitatedas previously described. The final mixture containing the platinum groupmetal may then be dried or calcined to provide a catalytically-activecomposition in a form suitable for deposition on a support or for usewithout such deposition as a finished catalyst in either finely dividedor macrosize forms. Subsequent calcinations or drying may be conductedas described above. The calcined material generally has a surface areaof at least about 25 s.m.p.g., preferably at least about 75 s.m.p.g.

The following are examples of the general method of preparation of somerepresentative stabilized catalytic composites and compositions of thisinvention. All percentages, parts, and proportions herein and in theappended claims are by weight unless otherwise indicated.

EXAMPLE I

A stabilized Cr₂ O₃, ZrO₂, and A1₂ O₃ composite slip is prepared bydissolving 3.95 grams of CrO₃ and 12.24 grams of zirconyl nitrate(49.03% as ZrO₂) in water and diluted to a total volume of 80.3 ml. 51grams of activated A1₂ O₃ powder is stirred into the solution withconstant agitation for 10 minutes. The total solution is then evaporatedto dryness under heat and with agitation, transferred to a drying ovenat 120° C., and dried overnight. Five grams of the composite containing5% chromia, 10% zirconia, and 85% alumina is then tested for retentionof surface area by calcining at 1200° C. for 4 hours. It is found thatthe surface area after such calcination is 35.3 m² /g.

EXAMPLE II

A 2 kilogram batch of the composite of EXAMPLE I is prepared in the samemanner except Cr(NO₃)₃ .sup.. 9H₂ O is used in place of CrO₃. The driedsolids are calcined for 4 hours at 1000° C. 186 grams of the calcinedpowder thus prepared are mixed with 286 ml. H₂ O and 13.9 ml. conc.HNO₃, and ball-milled for 19 hours at 68 RPM in a U.S. Stoneware1-gallon mill jar. 330 ml. of the resulting slip having a density of1.46 g/cc and a pH of 3.57 are diluted with 1% nitric acid to aviscosity of about 8 cps. A 1 inch × 3 inch zircon mullite honeycombhaving about 86 corrugations per square inch of cross-sectional area isdipped into the agitated, diluted slip, drained, blown with air, driedat 120° C. for 21/2 hours, and calcined at 500° C. for 2 hours. Theadherent composite makes up approximately 14.6 weight percent of thecoated honeycomb.

EXAMPLE III

A honeycomb, coated with the chromia-zirconia-alumina composite slip ofEXAMPLE I is prepared as in EXAMPLE II. The coated honeycomb is thendipped into a solution containing 18.00 grams of Na₂ PdCl₄ (35.10% Pd)dissolved in 51 cc water. After standing for 30 minutes withintermittent raising and lowering of the honeycomb into the solution,the honeycomb is withdrawn from the solution, drained, and excesssolution blown off. The honeycomb is then treated with 2% NaHCO₃ for onehour, heated in this solution of 95° C. for 15 minutes to accomplishhydrolysis. The coated honeycomb is then transferred to a sodium formatesolution and heated in this solution for reduction of the metal. Thecylinder is removed from the sodium formate, the excess solution blownoff and the cylinder is let stand overnight. The cylinder is then washedchloridefree using deionized water. The resulting impregnated honeycombis drained, dried overnight at 110° C. The finished catalyst containsabout 0.54 weight percent Pd.

EXAMPLE IV

A composite is prepared by coprecipitation. The composition is the sameas that is EXAMPLE I, i.e., 10 percent zirconia, 5 percent chromia, and85 percent alumina. 187.7 grams of aluminum nitrate, 6.1 grams ofzirconyl nitrate (49.03% as ZrO₂) and 7.9 grams of chromium nitrate aredissolved in one liter of water and the solution is transferred to adropping funnel. A second solution containing 400 ml of (28.3% NH₃)ammonium hydroxide and 1600 ml water was prepared in a dropping funnel.2500 ml of water is added to a 6 liter beaker with vigorous mechanicalstirring. The mixed nitrate solution is then added at room temperatureto the water in the beaker over a period of about 30 minutes. Theammonia solution is added concurrently with the nitrate solution at sucha rate as to keep the pH of the slurry in the beaker at 9.0. Stirring iscontinued for 15 minutes after the coprecipitation is complete. Theslurry is allowed to stand overnight, filtered, and reslurried in 2liters of water. The second slurry is filtered, excess water removed,and dried at room temperature. The filter cake is ground to a powder,dried for 1 day at room temperature, and overnight at 110° C. Thesurface area of the composite is good after calcination at 1200° C. for2 hours. Other oxide combinations of aluminum together with mixtures ofCr, W, and IVB metal oxides will be found to retain good surface areaafter calcination at 1200° C. for 2 hours.

In the practice of this invention the catalytic compositions areparticularly useful when emloyed with the high temperature oxidation ofcarbonaceous fuels. For example, they may be used advantageously in amethod employing a catalytically-supported thermal combustion ofcarbonaceous fuel, as more fully described in co-pending applicationSer. No. 358,411, filed May 8, 1973, of W. C. Pfefferle, now U.S. Pat.No. 3,928,961, issued Dec. 30, 1975, assigned to the assignee hereof andwhich application is incorporated by reference herein. This methodincludes the essentially adiabatic combustion of at least a portion of acarbonaceous fuel admixed with air in the presence of a catalyticcomposition of this invention at an operating temperature substantiallyabove the instantaneous autoignition temperature of the fuel-airadmixture but below a temperature that would result in any substantialformation of oxides of nitrogen.

Flammable mixtures of most fuels with air are normally such as to burnat relatively high temperatures, i.e., about 3300° F. and above, whichinherently results in the formation of substantial amounts of nitrogenoxides or NO_(x). However, little or no NO_(x) is formed in a systemwhich burns the fuel catalytically at relatively low temperatures.

For a true catalytic oxidation reaction, one can plot temperatureagainst rate of reaction. For any given catalyst and set of reactionconditions, as the temperature is initially increased, the reaction rateis also increased. This rate of increase is exponential withtemperature. As the temperature is raised further, the reaction ratethen passes through a transition zone where the limiting parametersdetermining reaction rate shift from catalytic to mass transfer. Whenthe catalytic rate increases to such an extent that the reactants cannotbe transferred to the catalytic surface fast enough to keep up with thecatalytic reaction rate, the reaction shifts to mass transfer control,and the observed reaction rate changes much less with furthertemperature increases. The reaction is then said to be mass transferlimited. In mass transfer ccontrolled catalytic reactions, one cannotdistinquish between a more active catalyst and a less active catalystbecause the intrinsic catalyst activity is not determinative of the rateof reaction. Regardless of any increase in catalytic activity above thatrequired for mass transfer control, a greater catalytic conversion ratecannot be achieved for the same set of conditions.

It has been discovered that it is possible to achieve essentiallyadiabatic combustion in the presence of a catalyst at a reaction ratemany times greater than the mass transfer limited rate. That is,catalyticallysupported, thermal combustion surmounts the mass transferlimitation. If the operating temperature of the catalyst is increasedsubstantially into the mass transfer limited region, the reaction rateagain begins to increase exponentially with temperature. This is anapparent contradiction of catalytic technology and the laws of masstransfer kinetics. The phenomena may be explained by the fact that thecatalyst surface and the gas layer near the catalyst surface are above atemperature at which thermal combustion occurs at a rate higher than thecatalytic rate, and the temperature of the catalyst surface is above theinstantaneous auto-ignition temperature of the fuel-air admixture(defined hereinbelow). The fuel molecules entering this layerspontaneously burn without transport to the catalyst surface. Ascombustion progresses, it is believed that the layer becomes deeper. Thetotal gas is ultimately raised to a temperature at which thermalreactions occur in the entire gas stream rather than only near thesurface of the catalyst. At this point, the thermal reactions continueeven without further contact of the gas with the catalyst as the gaspassed through the combustion zone.

The term "instantaneous auto-ignitiion temperature"for a fuel-airadmixture as used herein and in the appended claims is defined to meanthat the ignition lag of the fuel-air mixture entering the catalyst isnegligible relative to the residence time in the combustion zone of themixture undergoing combustion.

This method can employ an amount of fuel equivalent in heating value ofabout 300-1000 pounds of propane per hour per cubic foot of catalyst.There is no necessity of maintaining fuel-to-air ratios in the flammablerange, and consequently loss of combustion (flame-out) due to variationsin the fuel-to-air ratio is not as serious a problem as it is inconventional combustors.

The adiabatic flame temperature of fuel-air admixtures at any set ofconditions (e.g., initial temperature and, to a lesser extent, pressure)is established by the ratio of fuel to air. The admixtures utilized aregenerally within the inflammable range or are fuel-lean outside of theinflammable range, but there may be instances of a fuel-air admixturehaving no clearly defined inflammable range but nevertheless having atheoretical adiabatic flame temperature within the operating conditionsof the invention. The proportions of the fuel and air charged to thecombustion zone are typically such that there is a stoichiometric excessof oxygen based on complete conversion of the fuel to carbon dioxide andwater. Preferably, the free oxygen content is at least about 1.5 timesthe stoichiometric amount needed for complete combustion of the fuel.Although the method is described with particularity to air as thenon-fuel component, it is well understood that oxygen is the requiredelement to support proper combustion. Where desired, the oxygen contentof the non-fuel component can be varied and the term "air" as usedherein refers to the non-fuel components of the admixtures. The fuel-airadmixture fed to the combustion zone may have as low as 10 percent freeoxygen by volume or less, which may occur, for example, upon utilizationas a source of oxygen of a waste stream wherein a portion of this oxygenhas been reacted. In turbine operations, the weight ratio of air to fuelcharged to the combustion system is often above about 30.1 and someturbines are designed for air-to-fuel ratios of up about 200 or more:1.

The carbonaceous fuels may be gaseous or liquid at normal temperatureand pressure. Suitable hydrocarbon fuels may include, for example, lowmolecular weight aliphatic hydrocarbons such as methane, ethane,propane, butane, pentane; gasoline; aromatic hydrocarbons such asbenzene, toluene, ethylbenzene, xylene; naphtha; diesel fuel; jet fuel;other middle distillate fuels; hydrotreated heavier fuels; and the like.Among the other useful carbonaceous fuels are alcohols such as methanol,ethanol, isopropanol; ethers such as diethylether and aromatic etherssuch as ethylphenyl ether; and carbon monoxide. In burning diluted fuelscontaining inerts, for example, low BTU coal gas, fuel-air admixtureswith adiabatic flame temperatures within the range specified herein maybe either fuel rich or fuel lean. Where fuel rich mixtures are utilized,additional air or fuel-air admixture may be added to the catalyst zoneeffluent to provide an overall excess of air for complete combustion offuel components to carbon dioxide and water. As stated previously,thermal reactions continue beyond the catalyst zone, provided theeffluent temperature is substantially above the instantaneousauto-ignition temperature.

The fuel-air admixture is generally passed to the catalyst in thecombustion zone at a gas velocity prior to or at the inlet to thecatalyst in excess of the maximum flame propagating velocity. This maybe accomplished by increasing the air flow or by proper design of theinlet to a combustion chamber, e.g., restricting the size of theorifice. This avoids flashback that causes the formation of NO_(x).Preferably, this velocity is maintained adjacent to the catalyst inlet.Suitable linear gas velocities are usually above about three feet persecond, but it should be understood that considerably higher velocitiesmay be required depending upon such factors as temperature, pressure,and composition. At least a significant portion of the combustion occursin the catalytic zone and may be essentially flameless.

The carbonaceous fuel, which when burned with a stoichiometric amount ofair (atmospheric composition) at the combustion inlet temperatureusually has an adiabatic flame temperature of at least about 3300° F.,is combusted essentially adiabatically in the catalyst zone. Althoughthe instantaneous auto-ignition temperature of a typical fuel may bebelow about 2000° F., stable, adiabatic combustion of the fuel belowabout 3300° F. is extremely difficult to achieve in practical primarycombustion systems. It is for this reason that even with gas turbineslimited to operating temperatures of 2000° F., the primary combustion istypically at temperatures in excess of 4000° F. As stated above,combustion in this method is characterized by using a fuel-airadmixture, having an adiabatic flame temperature substantially above theinstantaneous atuo-ignition temperature of the admixture but below atemperature that would result in any substantial formation of NO_(x).The limits of this adiabatic flame temperature are governed largely byresidence time and pressure. Generally, adiabatic flame temperatures ofthe admixtures are in the range of about 1700° F. to 3200° F., andpreferably are about 2000° F. to 3000° F. Operating at a temperaturemuch in excess of 3200° F. results in the significant formation ofNO_(x) even at short contact times; this derogates from the advantagesof this invention vis-a-vis a conventional thermal system. A highertemperature within the defined range is desirable, however, because thesystem will require less catalyst and thermal reactions are an order ofmagnitude or more faster, but the adiabatic flame temperature employedcan depend on such factors as the desired composition of the effluentand the overall design of the system. It thus will be observed that afuel which would ordinarily burn at such a high temperature as to formNO_(x) is successfully combusted within the defined temperature rangewithout significant formation of NO_(x).

The catalyst used in this method generally operates at a temperatureapproximating the theoretical adiabatic flame temperature of thefuel-air admixture charged to the combustion zone. The entire catalystmay not be at these temperatures, but preferably a major portion, oressentially all, of the catalyst surface is at such operatingtemperatures. These temperatures are usually in the range of about1700°-3200° F., preferably about 2000° F. to about 3000° F. Thetemperature of the catalyst zone is controlled by controlling thecombustion of the fuel-air admixture, i.e., adiabatic flame temperature,as well as the uniformity of the mixture. Relatively higher energy fuelscan be admixed with larger amounts of air in order to maintain thedesired temperature in a combustion zone. At the higher end of thetemperature range, shorter residence times of the gas in the combustionzone appear to be desirable in order to lessen the chance of formingNO_(x).

The residence time is governed largely by temperature, pressure, andspace throughput; and generally is measured in milliseconds. Theresidence time of the gases in the catalytic combustion zone and anysubsequent thermal combustion zone may be below about 0.1 second,preferably below about 0.05 second. The gas space velocity may often be,for example, in the range of about 0.5 to 10 or more million cubic feetof total gas (standard temperature and pressure) per cubic foot of totalcombustion zone per hour. For a stationary turbine burning diesel fuel,typical residence times could be about 30 milliseconds or less; whereasin an automotive turbine engine burning gasoline, the typical residencetime may be about 5 milliseconds or less. The total residence time inthe combustion system should be sufficient to provide essentiallycomplete combustion of the fuel, but not so long as to result in theformation of NO_(x).

A method employing the catalysts of the present invention is exemplifiedin a test in which the fuel is essentially completely combusted, and alow emissions effluent is produced. The combustion system comprises asource of preheated air supplied under pressure. A portion of the air ispassed through a pipe to the combustion zone, and the remainder is usedto cool and dilute the combustion effluent. Unleaded gasoline fuel isatomized into the air passing to the combustion zone countercurrent tothe air flow to insure intimate mixing.

The catalyst is of the monolithic, honeycomb-type having a nominal6-inch diameter and is disposed within a metal housing as two separatepieces each having parallel flow channels 2 1/4 inches in lengthextendimg therethrough. There is a small space of about 1/4 inch betweenthese pieces. Both pieces of catalyst have approximately 100 flowchannels per square inch of cross-section with the walls of the channelshaving a thickness of 10 mils. The catalysts have similar compositionsand are composed of a zircon mullite honeycomb support which carries acomposite coating of alumina, chromia, and zirconia containingpalladium.

The catalyst for these runs is made by slurrying 3655 grams of activatedalumina powder, less than 40 mesh in size, in a mixer with a solutionprepared by dissolving 1131 grams Cr(NO₃)₃ .sup.. 9H₂ O and 877 gramszirconyl nitrate (49.03% as ZrO₂) in 1500 ml. H₂ O. The mixture is driedat 120° C. over a weekend. The dried solids are crushed and screened toless than 40 mesh, and the powder is calcined for four hours at 1000° C.to form the composite of this invention. 3000 grams of the composite ischarged to a 3 1/2 gallon ball mill along with 3000 ml. H₂ O and 145.4grams of palladium nitrate. The mill is rolled for 17 hours at 54 RPM.1600 grams of the as-recovered slurry are diluted with about 200 ml. ofa 1 percent nitric acid solution. The zircon mullite honeycomb is dippedin the diluted slurry and held for one minute, and then withdrawn fromthe slip and blown with air to remove the excess. The coated honeycombis dried for 16 hours at 110° C. and then calcined for 2 hours at 500°C. charged

The upstream or initial catalyst in the housing has a catalytic coatingwhich comprises about 14 weight percent of the catalyst. This coating is85 weight percent alumina, 5 weight percent Cr₂ O₃ and 10 weight percentZrO₂ based on these components. The catalyst also contains about 0.23weight percent palladium (calculated) disposed in the composite. Thesubsequent-in-line catalyst has a similar coating of alumina, zirconia,and chromia which is about 11.0 weight percent of the catalyst. Thecatalyst also contains about .16 weight percent palladium (calculated)disposed in the coating.

Provision is made for contacting the fuel mixed with a portion of thetotal air stream with the catalyst. That portion of the total air streamnot mixed with the fuel is added to the combustion effluent immediatelyupon its exit from the catalyst zone. This dilution or secondary aircools the combustion effluent and samples of the mixture are taken foranalysis. Thermocouples are located adjacent the initial catalyst inletand at the sampling position to detect the temperatures of theselocations.

The catalysts are brought to reaction temperature by contact withpreheated air, and subsequent contact with the air-fuel mixture whichcauses combustion and raised the catalyst temperature further. Theresults obtained using this system during operation in accordance withthe combustion method of the present invention are found to result inlow emission of pollutants.

The catalysts of this invention can also be used for selected oxidationreactions at lower temperatures. In a typical oxidation they can beemployed to promote the reaction of various chemical feedstocks bycontacting the feedstock or compound with the catalyst in the presenceof free oxygen preferably molecular oxygen. Although some oxidationreactions may occur at relatively low temperatures, many are conductedat elevated temperatures of about 150° C. to 900° C., and generally,these reactions occur with the feedstock in the vapor phase. The feedsgenerally are materials which are subject to oxidation and containcarbon, and may, therefore, be termed carbonaceous, whether they areorganic or inorganic in character. The catalysts of this invention areparticularly useful in promoting the oxidation of hydrocarbons,oxygen-containing organic components, for example, aldehydes, organicacids, and other intermediate products of combustion, such as carbonmonoxide, and the like. These materials are frequently present inexhaust gases from the combustion of carbonaceous fuels, and thus thecatalysts of the present invention are particularly useful in promotingthe oxidation of such materials thereby purifying the exhaust gases.Such oxidation can be accomplished by contacting the gas stream with thecatalyst and molecular or free oxygen. The oxygen may be present in thegas stream as part of the effluent, or may be added as air or in someother desired form having a greater or lesser oxygen concentration. Theproducts from such oxidation contain a greater weight ratio of oxygen tocarbon than in the material subjected to oxidation and in the case ofexhaust purification these final oxidation products are much lessharmful than the partially oxidized materials. Many such reactionsystems are known in the art.

What is claimed is:
 1. A method for the combustion of carbonaceous fuelcomprising: forming an intimate admixture of said fuel and air;contacting said fuel air admixture with an oxidation catalyst at atemperature sufficient to combust said admixture, said catalyst having asurface area of at least 20 m² /g after calcination for two hours at atemperature of 1200° C., said catalyst comprising: (a) acatalytically-active calcined composite of alumina and a mixture of twometal oxide components wherein the first component is selected from thegroup consisting of an oxide of Cr, W, and mixtures thereof and thesecond component is selected from the group consisting of an oxide of aGroup IVB metal and mixtures thereof and (b) a catalytically-effectiveamount of platinum group metal deposited thereon after calcination ofsaid composite at a temperature of at least 850° C.
 2. A method asdefined in claim 1 wherein said combustion is catalytically-supportedthermal combustion forming an effluent of high thermal energy said fuelbeing in vaporous form and intimately admixed with air; said combustionbeing under essentially adiabatic conditions and being characterized bysaid fuel-air admixture having an adiabatic flame temperature such thatupon contact with said catalyst, the operating temperature of saidcatalyst is substantially above the instantaneous auto-ignitiontemperature of said fuel-air admixture but below a temperature thatwould result in any substantial formation of oxides of nitrogencomprising: contacting said fuel-air admixture with an oxidationcatalyst having a surface area of at least 20 m² /g after calcinationfor two hours at a temperature of 1200° C. comprising: (a) acatalytically-active calcined composite of alumina, and a mixture of twometal oxide components wherein the first component is selected from thegroup consisting of an oxide of Cr, W, and mixtures thereof and thesecond component is selected from the group consisting of an oxide of aGroup IVB metal, and mixtures thereof and (b) a catalyticaly-effectiveamount of platinum group metal deposited thereon after calcination ofsaid composite at a temperature of at least 850° C.
 3. A method asdefined in claim 2 further comprising depositing said composite on arelatively inert substrate to form a coating thereon prior to saidplatinum group metal deposition.
 4. A method as defined in claim 3wherein said substrate is a honeycomb.